Power converters typically include power switches in the form of power transistors that are connected in a Darlington configuration and rapidly switched on and off. Usually, these transistors are driven into saturation so that the voltage across the main current path electrodes thereof is maintained at a low level. This, in turn, minimizes the amount of power dissipated by the transistors. However, when the load current conducted by the transistors increases beyond a certain magnitude, as determined by the transistor base current, the transistors are pulled out of saturation resulting in a relatively high level of power dissipation. In extreme circumstances, the transistors can experience thermal runaway, leading to destruction thereof.
One type of circuit which avoids the foregoing difficulty is known as a proportional base drive circuit wherein a transformer is connected between the collector of an output transistor and a base drive circuit connected to a base of the transistor. The base drive circuit adjusts the magnitude of base current supplied to the output transformer to maintain the transistor in saturation. Such a base drive circuit is disclosed in Shuey, et al., U.S. Pat. No. 4,453,089.
One disadvantage of the proportional base drive circuit is the need for the transformer which unduly increases the size and weight of the overall circuit. This can, in turn, render the overall power converter unsuitable for certain applications, such as in aircraft, where size and weight must be kept to minimum levels.
Japanese patent provisional publication no. 57-188133 discloses a transistor relay circuit including a main transistor, a driver transistor coupled to the main transistor in a Darlington configuration and an NMOS transistor having a drain electrode coupled to the collector electrodes of the driver and main transistor and a source electrode coupled to the base of the driver transistor. Drive signals are provided directly to a gate electrode of the NMOS transistors and to the base of the main transistor via a base resistance. During normal operation when main transistor collector current is at or below a certain magnitude, sufficient base current is supplied to the main transistor to maintain it in a saturated condition. The driver and NMOS transistor are off at this time. When the current flowing through the collector and emitter of the main transistor rises above the certain magnitude during peak current conditions, the main transistor pulls slightly out of saturation, in turn applying enough voltage to the NMOS transistor and the driver transistor so that they turn on. These transistors then supply sufficient additional current to the main transistor base to cause the voltage across the collector and emitter of the main transistor to be reduced. This circuit, therefore, limits base drive current during normal or steady state operating conditions yet provides the capability for additional base drive during peak conditions.
When the circuit disclosed in the Japanese publication referred to above is used to operate a large main transistor, the base drive power required to turn on the main transistor is excessive due to the presence of the base resistor and the fact that the driver transistor is not utilized under normal conditions. In addition, since the driver transistor is not normally conducting load current, the main transistor may be driven into hard saturation when lightly loaded. This results in long storage times when the main transistor is turned off leading to delays that adversely affect the function of the circuit.
Ickes, et al., U.S. Pat. No. 4,055,794 discloses a circuit for regulating the base drive voltage of an output transistor. The voltage across the collector and emitter of the output transistor is detected and a signal representative thereof is compared by a comparator against a reference. The comparator in turn controls a base drive circuit coupled to the driver transistor base whereby the output transistor is operated just at saturation.
Patents disclosing Darlington-connected transistors and biasing or drive circuitry for such transistors include Ullman, U.S. Pat No. 3,210,561, Nawa, et al., U.S. Pat. No. 4,138,690, Tani, U.S. Pat. No. 4,769,560 and Shekhawat, et al., U.S. Pat. No. 4,725,741, assigned to the assignee of the present application.